In the current art, a number of systems and methodologies exist for localization of aircraft and ground vehicle targets for Air Traffic Control. These systems provide airport surface, airport terminal area and en-route surveillance.
Primary radar and Secondary Surveillance Radar (SSR) systems are widely used to provide surveillance for Air Traffic Control. Radars use centralized sensor architecture, using a single rotating antenna to provide surveillance. Surveillance position data from several radar systems with overlapping coverage can be fused to provide surveillance over wide coverage areas. Each SSR provides high accuracy target-ranging capability and target angle measurement that degrades with target distance from the SSR. The SSRs are not synchronized to the timing accuracies required to perform precision surveillance via triangulating position between two or more SSRs. Expanding surveillance coverage area requires installing additional radar systems.
A radar system uses a large rotating antenna, which make radars both expensive to acquire and to maintain. Radars are susceptible to false targets in high multi-path environments. These false targets are difficult to detect and eliminate in the surveillance processing. Radar integrity monitoring is supported by fixed radar reflectors or parrot transponders, which are located at known positions within surveillance range of each radar. Failure of a radar to locate a fixed radar reflector or parrot transponder at the known position results in the identification of a system integrity failure. A small number of fixed radar reflectors or transponder parrots are required due to the centralized radar system architecture of a single rotating antenna per radar.
Multilateration systems are used as either an alternative to radar systems or to augment radar system surveillance. Multilateration systems use distributed sensor architecture, whereby a surveillance system will consists of a central workstation and a network of three or more sensors that are geographically separated. The sensors can be configured as receive only, transmit only and receive/transmit. Each sensor is housed in a small electronics enclosure and uses fixed antenna(s). Expanding surveillance coverage requires adding sensors to an existing multilateration system, as opposed to the more expensive route of installing an additional system.
Multilateration systems can be configured for a low probability of false target in high multi-path environments. However, multilateration systems can benefit from new methods to reduce the risk of false targets. Integrity monitoring is supported by reference transmitters or transponders as defined in Minimum Operational Performance Specification for Mode-S Multilateration Systems for use in Advanced Surface Movements Guidance and Control Systems (A-SMGCS), ED-117, incorporated herein by reference, which are located at known positions within surveillance coverage area of the system.
Failure of the multilateration system to locate the fixed reference transmitter or transponder at the known position results in the identification of a system integrity failure. Multilateration system monitoring has been implemented such that that each reference transmitter or transponder is in view of two or more multilateration sensors. However, this monitoring implementation is not practical when the multilateration sensors are separated by more than five kilometers. Terrain and radio horizon constraints require that the reference transmitter or transponder antenna be located high enough to be in view of the multilateration sensors. In some cases, these high sites are either not available or require expensive antenna tower installations.
Thus, it remains a requirement in the art to provide an improved technique for integrity monitoring of multilateration systems, which is relatively inexpensive and easier to implement.